Is it illegal to slander someone over the internet on a "slander" site?

I did not do this but someone I know got on a website where people post bad stuff about a man or woman warning people to stay away from them. One of the men who was badmouthed found the post , knows who posted it and is threatening a lawsuit. Is what the slanderer did illegal? The slandered man is not a public figure by any means but is wealthy and in his eighties.